Add smp_copy_signspace() and smp_trunc_signspace() utility functions

smp_copy_signspace() will copy the sign data of one space onto another.

smp_trunc_signspace() will truncate the signed space to len bytes and
resign the space.

bin/varnishd/storage/storage_persistent.h

@@ -207,6 +207,9 @@ void smp_def_signspace(const struct smp_sc *sc, struct smp_signspace *spc,
 int smp_chk_signspace(struct smp_signspace *spc);
 void smp_append_signspace(struct smp_signspace *spc, uint32_t len);
 void smp_reset_signspace(struct smp_signspace *spc);
+void smp_copy_signspace(struct smp_signspace *dst,
+			const struct smp_signspace *src);
+void smp_trunc_signspace(struct smp_signspace *spc, uint32_t len);
 
 void smp_newsilo(struct smp_sc *sc);
 int smp_valid_silo(struct smp_sc *sc);

bin/varnishd/storage/storage_persistent_subr.c

@@ -223,6 +223,37 @@ smp_reset_signspace(struct smp_signspace *spc)
 	smp_reset_sign(&spc->ctx);
 }
 
+/*--------------------------------------------------------------------
+ * Copy the contents of one signspace to another. Prepare for
+ * appending.
+ */
+
+void
+smp_copy_signspace(struct smp_signspace *dst, const struct smp_signspace *src)
+{
+	assert(SIGNSPACE_LEN(src) <= dst->size);
+	smp_reset_signspace(dst);
+	memcpy(SIGNSPACE_DATA(dst), SIGNSPACE_DATA(src), SIGNSPACE_LEN(src));
+	smp_append_signspace(dst, SIGNSPACE_LEN(src));
+	assert(SIGNSPACE_LEN(src) == SIGNSPACE_LEN(dst));
+}
+
+/*--------------------------------------------------------------------
+ * Reapplies the sign over the len first bytes of the
+ * signspace. Prepares for appending.
+ */
+
+void
+smp_trunc_signspace(struct smp_signspace *spc, uint32_t len)
+{
+	assert(len <= SIGNSPACE_LEN(spc));
+	spc->ctx.ss->length = 0;
+	SHA256_Init(&spc->ctx.ctx);
+	SHA256_Update(&spc->ctx.ctx, spc->ctx.ss,
+		      offsetof(struct smp_sign, length));
+	smp_append_signspace(spc, len);
+}
+
 /*--------------------------------------------------------------------
  * Create a new signature space and force the signature to backing store.
  */